Compact precision measuring head, which is resistant to high-pressure, for measuring the optical refractive index in liquids

ABSTRACT

The invention relates to a compact precision measuring head for measuring the refractive index in liquids and gases with an extremely high degree of accuracy, under environmental pressures of 1000 bar and greater. The measuring head consists of a single-block ( 1 ) glass reference prism connected to a stable high-pressure bulkhead fitting ( 2 ) which serves as the linking element between the measuring medium and the pressure-resistant interior of the instrument housing, said housing containing the optoelectronic assemblies for generating the electric measuring signals for the subsequent calculation of the desired refractive index values. The measuring head is designed in particular for economic, automatic mass production on computerized machines. Absolute calibration takes place using normal liquids and/or gases to an accuracy of approximately 10 −6 . . . 10 −7 .

[0001] For a long time, refractometers have been widely used todetermine the optical refractive index in liquids and gases in which inthe most diverse forms of the device the exit angle of the refractedlight ray in its transition from the measurement medium to a referencemedium in the direction of the perpendicular is determinedquantitatively. The basis of these measuring instruments is Snellius'Law of Optical Refraction. The achievable accuracy of the refractiveindex of the medium being studied is thus dependent, among otherfactors, on the accuracy of the optical refractive index of thereference body and of the angles α and β of the incident and therefracted light ray, as well as on certain characteristics of the lightsource and of the detector measuring the incoming ray. In order toachieve maximum accuracy in measurements it is necessary to fulfill thehighest stability requirements, especially in some optical/mechanicalcomponents and their alignment in the optical bank of the measurementinstrument. The absolute values of the angles, the measurements and therefractive index of the reference body must not necessarily be knownvery exactly, since the refractometer can be calibrated using one ormore liquids and/or gases the precise optical refractive index of whichis/are exactly known.

[0002] Refractometers of especially high accuracy in in situmeasurements in the ocean, as well as in the laboratory, have asignificant role in determining the physical state quantities of oceanwater, especially in the extensive spaces of the deep sea. Thus for along time attempts have been made to create appropriate instruments forfield use, but the stability and/or accuracy achieved has remainedsomewhat unsatisfactory. Nonetheless, refractive index accuracy in therange of 10⁻⁶, possibly even to 10⁻⁷, is required for meaningfulrefractive index measurement in the ocean, which means that in practicerefractive angle measurements which are stable and maximally long-termconstant in the magnitude of one-tenth arc second must be achieved athydrostatic environmental pressures up to ca. 1000 bar.

[0003] To date it could be shown that such high stability requirementsare basically achievable, as for example evident in recently describedand experimentally tested field instruments. (OCEANS '88, IEEE Publ. No.88-CH 2585-8, Baltimore, Md., USA, Volume 2(4), 497 . . . 504, (1988);OCEANS '99, MTS/IEEE Publication, Seattle, Wash., USA, ISBN:0-933957-24-6, Vol. 3, 1218 . . . 1222, (1999)).

[0004] In order to be able to classify the inventive object exactly, thebasic refractometer principle will be described briefly here, with theassistance of FIG. 1. At first glance this is an instrument which in itsexterior dimensions such as length and diameter is comparable to thewell-known, classical Abbé submersible refractometer. The rod-shapedmeasuring device has in its one end the sensor measuring head A to besubmerged in the liquid (or gas) to be tested, and a housing Ccontaining the optical bank, light source and sensor electronics. Themeasuring head and the housing wall are separated by a high-pressurebulkhead fitting B, through which especially the optical measurementsignals can be conducted in a highly stable manner without, for example,experiencing an uncontrolled influence of the measurement value and thusan unacceptable reduction of measurement accuracy due to compression.For all measurements the device can be completely submerged in themedium. The necessary supply and signal connections from the sensorinterior to the outside and vice versa can, for example, be achieved bymeans of pressure resistant plugs or cable fittings or also, forexample, via magnetic or optical transmission paths through the housingwall to the extent the housing material has no magnetic or opticalshortcomings. Pressure resistant glass or ceramic housings are examples.

[0005] For a description of the measurement process refer to FIGS. 1, 2and 3. FIG. 1 shows the functional principle of the refractometer inschematically represented blocks, FIG. 2 the path of the rays withreference to the refractive index prism in which ε indicates the prismangle between the light entry plane at the boundary between themeasurement medium and reference prism and the plane of the seat of theprism on the high-pressure bulkhead fitting. FIG. 3 illustrates how, forexample, unnecessary areas of the reference prism can be eliminated bymeans of grinding in order to improve hydrodynamic rinsing of themeasurement area of the sensor without influencing the active measuringsurfaces and without compromising the pressure resistance. Well-definedlight emanating from light source 1 is divided, for example, in anoptical fiber coupler 2. Part of the light is passed via a lightwaveguide through the high-pressure bulkhead fitting B into thecollimator 3 where it is bundled to a fine collimated light beam. Afterpassing through the measurement medium it strikes the refractive indexreference prism 4 under the angle of incidence α, is refracted in thedirection of the perpendicular under the angle β, finally passes againthrough the high-pressure bulkhead fitting into the pressure-protectedinner housing and reaches the photoelectric receiver 5, which permitsextremely exact determination of the refractive angle. The variousphotoelectric currents of the light converter are converted with the aidof the transimpedance amplifier 6 and 7 into highly precise electricvoltages. From these voltages it is possible, in the known manneraccording to the state of the art, to calculate the refractive index ofthe tested medium electronically and fully automatically to ca. 10⁻⁷after the coefficients of the conversion algorithm have been determinedin a previous calibration process. For exact monitoring of the lightsources used, or their light wavelengths, another part of the opticalwave fork coupler 2 is led into the light wavelength measurement module10, which supplies a calibrated electrical voltage U_(λ), from which thewavelength can be explicitly calculated and then in appropriate mannerused for the mathematical determination of the refractive index. In thesame manner, fine correction of the residual influences of pressure andtemperature on the measurement head can be made by separate measurementof these magnitudes on the reference prism on the high-pressure bulkheadfitting. For this purpose there is a thermometer 8 mounted on themeasurement head, which makes available the calibrated measurementvoltage U_(T) for subsequent data processing. Ambient pressure isdetermined by means of a separate pressure gauge.

[0006] According to this basic principle, absolute refractive indexaccuracy between 10⁻⁶ and 10⁻⁷ in situ is routinely possible if there isa corresponding high-pressure resistant, long-term stable precisionmeasurement head with a reference prism. The versions of the sensor headrealized to date, which have been tested in the ocean and in thelaboratory, were comprised of numerous costly individual parts whichposed significant and difficult to fulfill demands with regard topressure resistance, stability and accuracy, which were moreover verycostly to assemble, adjust and align, and for which costly, customadjustment devices and measurement instruments were required, so that onthe whole the previous measuring heads were very critical in theirmanufacture and subsequent maintenance of accuracy. In addition,production costs were sufficiently high that the utilization of largenumbers of them in networks in the ocean in so-called expendable,one-way probes did not appear at all feasible.

[0007] The invention described here has to do with a completely newsolution of the sensor head problem by forming the critical sensor partfrom a single homogenous optical material which can be manufacturedcompletely in automatic machine production and contains no foreign partsrequiring adjustment. The typical form of this so-called “single block”sensor head is in principle shown in FIG. 4: the view shows a mainsection through the reference prism body 1 and high-pressure bulkheadfitting 2, which are firmly connected with each other andpressure-sealed. The fine collimated light beam (for example 0.5 mmdiameter), which comes out of the pressure-protected interior cavity ofthe instrument through a boring in the bulkhead fitting, is reflected onthe mirroring surface 3 in such a manner that it exits surface 4vertically into the measurement medium and after a short distancestrikes the entrance surface 5, where it is refracted into the referenceprism to the perpendicular 6. On the additional mirroring surface 7 thebeam is then reflected again into the interior of the instrument throughthe corresponding boring in the high-pressure bulkhead fitting and therefinally reaches the detector measuring the refractive angle.Individually it is possible to mirror coat the surfaces 3 and/or 7 or toachieve the reflection by means of total reflection on the correspondingsurfaces.

[0008] The part of the sensor head thus comprised of a single glassmaterial and possibly having, for example, applied mirror coatings ofgold on the indicated optical parts, can be manufactured completelymechanically by sawing, grinding, polishing and possibly mirror coatingfollowing prior mathematical determination of the collective angles,surfaces and measurements, and according to the selection of theappropriate optical glass material and determination of the measurementrange appropriate for the liquids or gases to be measured. Regardless ofwhether total reflectivity on the relevant mirror surfaces is achievedor not, gold facings can, for example, be applied for additionalprotection of the surface with regard to even the smallest mechanical orchemical blemishes, since given the extremely high measurement precisioneven the smallest imperfections can lead to a significant displacementof the focal intensity of the light ray, which then would be a cause formeasurement deviations.

[0009] In very many applications, especially in oceanography, streamingis from the sensor head side toward the sensor in the direction of theinstrument axis. To optimize the streaming behavior, especially for thepurpose of faster, particularly more effective free rinsing of themeasurement volume in the light ray between the surfaces 4 and 5 in FIG.4, the shadow-free slant position shown here of these two surfaces isselected and free grindings are simultaneously undertaken to remove asmuch of the prism material as possible so that optimal streamingbehavior and the fastest possible free rinsing in the area of themeasurement volume are achieved, without however compromising therequired mechanical stability of the measurement head and withoutdisturbing the measurement ray in the interior of the glass body or inthe area of the measurement volume of the medium to be tested.

[0010]FIG. 5 shows the reference prism of the sensor head comprised of acircular cylinder with the ground fastening groove a, which allows theprism to arrange with reference to the upper cylinder area b and thelower cylinder area c. The latter facilitates problem-free O-ringhigh-pressure sealing of the glass body on the high-pressure bulkheadfitting part 2 of FIG. 4, described below. The upper cylinder part b canbe used as sealing surface for a protective cover turned back onto thesensor or for a capsule-like container with preservative and/orcalibration liquid.

[0011]FIG. 6 is a side view of the prism in FIG. 5 seen turned 90degrees from the left, while FIG. 7 is a vertical view from above. Inthese illustrations it is shown, for example, how hydrodynamicimperfections can be ground away, whereby an optimum betweenstreaming-favorable form and mechanical stability can be achieved. Theespecially critical area d in FIG. 5, directly at the place where theoptical fiber enters the measurement medium, must be very exactlycontrolled, for example made with a narrow saw blade as a simple groovecut, in order to form in a simple manner the less critical slopingsurfaces around the elliptical area of the optically used refractivesurface e (see FIG. 8).

[0012]FIGS. 8 and 9 are spatial sketches of the upper part b of FIG. 5for a better visualization of the streaming-optimized design of the“single block” sensor head. FIG. 9 shows especially the path of the rayas well as any surfaces which may be plated e.g., with gold Au.

[0013] The refractive index reference body is provided with a polishedflat surface in its lower part c according to FIG. 5, which rests as ahigh-pressure resistant, planar, undislodgable base on the planarsurface f of the high-pressure bulkhead fitting according to FIG. 10.Spring tabs g press downward on the glass body in the direction of theinstrument axis. A precisely-fitting centering ring h protects againstradial displacement. The clamping ring i is, for example, firmlyconnected by threading to the main body 1 of the high-pressure bulkheadfitting and has O-ring seals k and j, which are durable with referenceto the medium to be measured.

[0014] The entire unit of the high-pressure resistant compact precisionmeasuring head for highly exact optical refractive index measurements ineither at rest or flowing liquids and for gases is shown in FIG. 11. Aversion has been produced according to FIG. 11 with a glass cylinderdiameter of 27 mm. Function and high-pressure resistance weredemonstrated.

[0015] Finally, it is to be noted that according to the type ofapplication the space between the surfaces 4 and 5 in FIG. 4 can bevariously formed. There is a given constant measurement volume withconstant elliptical cross-section of the entry ray through surface 5into the measurement prism, especially in the case of the vertical exitof the measurement ray through surface 4 into the measurement medium. Inthis case the angle between the surfaces 4 and 5 is smaller than 90degrees.

[0016] One can however also increase the angle between the surfaces 4and 5 to more than 90 degrees in order, for example, to increase themeasurement sensitivity, i.e. to increase the change of the entiredeflection angle with the change of the refractive index in the fluid,in order then to let the measurement ray pass through both surfacesobliquely, which however somewhat changes the measurement volumeaccording to location and form and especially also the elliptical crosssection of the light ray as it passes through surface 5 according toposition and form, if the refractive index of the measurement mediumchanges. This means that a somewhat differently situated area of themeasurement prism is utilized.

[0017] For the production of the homogenous glass prism it is to benoted that it can be made from a single work piece but also can be puttogether from several segments, which then for example are diffusionwelded, so that optical and also mechanical homogeneity are completelyassured.

1.) High-pressure resistant compact precision measurement head forhighly exact optical refractive index measurements in liquids and/orgases characterized by a refractive index reference body, mounted on ahigh-pressure bulkhead fitting, which body is composed of a single,optically homogenous material produced by pre-calculation of itsgeometry economically, mechanically and automatically with extremely lowtolerances. The measurement head can be formed in such a manner that ithas on its front portion an especially small measurement volume to aslittle as less than 0.5 mm³, through which a very thin opticalmeasurement ray passes. The latter is produced behind the high-pressurebulkhead fitting in a pressure-protected interior cavity of theinstrument and also evaluated there as an incoming ray after passingthrough the measurement medium. The precision measurement head possessesthereby no assembly and adjustment elements of optical components in thehigh-pressure area of the measurement medium. 2.) High-pressureresistant compact precision measurement head according to claim 1.)characterized in that the ray exiting the measurement medium exitseither vertically, so that a refraction occurs only on the oppositesurface and thus the measurement volume remains constant for allrefractive indices in the liquids and/or gases to be measured, or thatthe measurement ray enters obliquely into the measurement medium so thatnow the position and size of the measurement volume depend on therefractive index value to be measured, however thereby a higher totaldeflection sensitivity arises for the refracted light ray. In this caseangles can be selected which are clearly greater than 90 degrees betweenthe refracting surfaces. 3.) High-pressure resistant compact precisionmeasurement head according to claims 1.) and 2.) characterized in thatits refractive index reference body has sensitive measurement surfacesat its point which are especially small, planar, polished and sopositioned that there is no shadowing while streaming the measurementmedium from forward along the axis of the measurement instrument. 4.)High-pressure resistant compact precision measurement head according toclaims 1.), 2.), 3.) characterized in that for its refractive indexreference body a high optical constant and mechanical stability of theposition of the measurement surfaces is achieved even under theinfluence of very high static and dynamic forces, such that on a single,firm, very compact homogenous block small measurement surfaces, lyingvery close to each other, are realized. Thereby thesemeasurement-sensitive surfaces can be provided opposite its edge withsloping bevels in an optimal manner, so that excellent characteristicsin fast streamings and high speed profile measurements in the deep seacan be achieved while maintaining the necessary stability. 5.)High-pressure resistant compact precision measurement head according toclaims 1.) through 4.) characterized in that its refractive indexreference body has a planar surface as base on the high-pressurebulkhead fitting, as well as a cylinder surface for very simple andhighly reliable O-ring high-pressure sealing. 6.) High-pressureresistant compact precision measurement head according to claims 1.)through 5.) characterized in that its refractive index reference bodycan have a small groove on its cylinder surface, through which thefixing on the high-pressure bulkhead fitting can be achieved with smallpressure claws, whereby a precisely fitting centering ring preventsradial displacement. 7.) High-pressure resistant compact precisionmeasurement head according to claims 1.) through 6.) characterized inthat its refractive index reference body has an additional cylindersurface above a fastening groove, which can be used as a highlyeffective sealing surface, for example, a cap for the measurement point,which can be done as a simple protective cap or also as an ampoule-likecontainer for preservation liquid or for gas or particularly referenceliquid or gas. 8.) High-pressure resistant compact precision measurementhead according to claims 1.) through 7.) characterized in that therequired deflection surfaces of the refractive index reference body areeither totally reflective or are plated with a mirror coating fordeflection of the light ray prior to entry into the measurement volumeand after exit from the same by means of selection of the correspondingangle of the mirror surfaces and of the refractive index of the materialof the reference body; the coating can be, for example, of chemicallyespecially durable gold and can also be utilized simultaneously asmechanical or chemical protection. 9.) Device for optical refractiveindex measurement up to 10⁻⁷ in flowing or still liquids or gases atpressures up to 1000 o/bar, comprising a high-pressure resistant,compact sensor measurement head (A), with a refractive index referencebody (4) and a housing (C) separated by a high-pressure bulkhead fitting(B), containing an optical bank, a light source and electronics,characterized in that its refractive index reference body (4), which ismounted on the high-pressure bulkhead fitting (B) is a single optical,monolithic material block and can be produced automatically,mechanically with a tolerance in the sensitive surface of <70/100,whereby the measurement head (A) is so formed that it has a measurementmedium in its front part with a volume up to <0.5 mm³ for a finecollimated light beam running therethrough, without any assembly andadjustment elements of optical components in the high-pressure region ofthe measurement medium. 10.) Use of the precision measurement headaccording to claim 1 through claim 8 or of the device according to claim9, for large-scale use in large measurement networks in the ocean,especially deep-sea utilization, for monitoring changes in physicalstate properties of the water.